The present invention is directed to electrically conductive adhesives for the planar joining of corresponding planar parts to one another.
Adhesives are used in the manufacture of electric surface wave filters, large-area IC semiconductor circuits, and corresponding electronic components in order to glue the components to a substrate. For example, if the substrate is to be a component part of the housing of the finished component, it is necessary to glue the substrate of, for example, the wave filter or, respectively, of the IC circuit or the like on, for example, a metallic substrate.
With respect to the substrates, it is known, for example, to utilize as a substrate for wave filters, laminae of single-crystal lithium niobate or tantalate having a length of up to several cm long and being about 0.5 mm thick. Substrate laminae of silicon or of other semiconductor material as well are used for IC circuits. These substrate materials share some similar properties that influence their use. Specifically, these materials are relatively sensitive to mechanical stresses and tend to break or rip when subjected to such mechanical stresses. Particularly in the case of surface wave filters, there is a further problem that the substrates are vulnerable to even mere mechanical deformations. A mere mechanical deformation, particularly bending, of the substrate leads to extremely great modifications in the electrical values of the component.
It is known to create a surface wave filter, or, respectively, an IC circuit on or, respectively, in a surface of the substrate. The substrate is connected to a foundation on an opposite surface thereof with an adhesive. The foundation is electrically grounded and forms an electrical shielding for the electronic structure of the filter or, respectively, of the IC circuit on the former surface. However, it is often important for such components that this shielding lying at ground, lie as close as possible to the zone of the electrical structure. When electrically conductive adhesive or glue having adequately high electrical conductivity for this purpose is utilized, then this shielding lies at that surface of the substrate lying opposite the electronic structure, i.e., considerably closer to the respective electronic structure.
It is preferable that the adhesive layer is sufficiently thick so that it absorbs all mechanical stresses to the greatest extent possible. Such mechanical stresses are due to, on the one hand, the different temperature coefficients of expansion of the material of the substrate and, on the other hand, on the material of the foundation. The adhesive layer absorbs these mechanical stresses because of the elasticity of, or, respectively, the viscosity of, the adhesive.
It is known in the art to use epoxy silver glue as an adhesive for these components. Epoxy silver glue provides satisfactory results as an adhesive for such components in temperature ranges between approximately 0.degree. C. to about 125.degree. C. However, there are numerous applications for these components in which a considerably broader range of temperatures of electrical operation or at least of storing the components must be taken into consideration. A temperature range that is typically required extends from approximately -40.degree. C. to about 180.degree. C.
In this broader temperature range, difficulties are encountered using typical adhesives. This is particularly true at lower temperatures at which an epoxy adhesive is so hard that it does not exhibit the required elasticity necessary to compensate for the stresses occurring within the adhesive. Because epoxy adhesives do not have the required elasticity at these lower temperatures, if epoxy adhesives are utilized, at these lower temperatures the substrate cracks and the component becomes unusable. The disadvantages of these known typical adhesives become especially acute in electronic applications, wherein even thermally induced bending of the substrate is not acceptable.
Accordingly, there is a need for an electrically conductive adhesive that is useable over a broader temperature range.